Field of the Invention
The present disclosure relates to a multilayer actuator, a display device comprising the same, and a method of manufacturing the same.
Discussion of the Related Art
Recently, as users desire to conveniently use various display devices such as liquid crystal display (LCD) devices and organic light emitting diode (OELD) displays, the use of touch-type display devices has become common. In this regard, researches have been continuously conducted on an actuator in order to provide direct and various touch feedbacks to users. In addition, researches have been conducted to implement various displacements of a flexible display panel by attaching an actuator to the flexible display panel.
In general, a conventional display device has employed a vibration motor such as an eccentric rotating mass vibration motor (ERM) or a linear resonance actuator (LRA) as an actuator. The vibration motor is designed to vibrate the whole display device, and thus has a problem in that a mass body needs to be increased in size to increase its vibration power. In addition, the vibration motor has disadvantages in that frequency modulation for adjusting a level of vibration is difficult, a response speed is significantly low, and the vibration motor is not suitably used for the flexible display device.
To address these issues, a shape memory alloy (SMA) and electro-active ceramics (EAC) have been developed as materials of the actuator. However, the SMA has a low response speed and a short lifespan, and is opaque, and the EAC is fragile. Thus, there has been a difficulty in applying the SMA and the EAC to a display device, in particular, a flexible display device.
In this regard, an actuator technology using an electro-active polymer (EAP) has been receiving attention in the industry. The EAP refers to a polymer that can be deformed by electrical stimulation, and refers to a polymer that can repeatedly expand, contract and bend by electrical stimulation. Researches have been conducted to manufacture an actuator comprising the EAP as an electroactive layer, and such an actuator is attached to a flexible display panel, thereby implementing diverse bending of a flexible display.
However, bending ability of an actuator comprising only one electroactive layer is restricted due to its increased thickness and high driving voltage. To address these problems, a multilayer actuator configured by stacking a plurality of unit actuators, each of which comprises one electroactive layer, has been introduced. Such a multilayer actuator comprising a plurality of electroactive layers can implement a higher driving displacement at the same thickness when compared to an actuator comprising only one electroactive layer.
An electroactive layer of a unit actuator in a conventional multilayer actuator typically comprises a dielectric elastomer. The dielectric elastomer does not have polarization in a natural state, and thus, previous researches have not considered polarizations of a plurality of electroactive layers. As a result, previous researches have not considered a form of arrangement of polarization directions of a plurality of electroactive layers.
Recently, a ferroelectric polymer, which can ensure a higher driving displacement when compared to the dielectric elastomer, has been implemented as an electroactive layer. The ferroelectric polymer has a natural polarization in a specific direction, and thus, there is a need for considering a form of arrangement of polarization directions of a plurality of electroactive layers comprising the ferroelectric polymer in a multilayer actuator. In this regard, some methods have been introduced in which a direction of an applied electric field for each of the plurality of electroactive layers is adjusted based on expansion and contraction of the electroactive layers according to the direction of the applied electric field. However, significant effectiveness has not been achieved so far.